provisioner/ansible: configurable command / args

allow users to provide a replacement command / extra args to the
ansible-local provisioner.

this is useful, for example, to unbuffer output when using the
docker builder, or to modify ansible behavior, like enabling
verbose mode, injecting variables, or changing paths.
provisioner/ansible: support {group,host}_vars

ansible looks for group and host vars directories as well as playbooks
this revision to the provisioner uploads those as well.
squashed to a single commit by strcrzy.
Merge pull request #842 from strcrzy/ansible_config_options

provisioner/ansible: more configuration options